Chapter 10 (page 40)

The Pyloric Sphincteric Cylinder in Health and Disease

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Chapter 10 (page 40)

Chapter 10

Ultrasonography of Normal Anatomy

Conventional surface Ultrasonography of the Normal Infantile Pylorus

The sonographic appearance of infantile hypertrophic pyloric stenosis (IHPS) was first described by Teele and Smith (l977) (Chap. 23). Subsequently a number of authors investigated the sonographic features of the normal infantile pylorus.

In transverse section the normal pylorus presents as a hypoechoic ring with a central echogenic core, resembling a "doughnut" or "target" (Fig. 10.1). By comparing sonographic images with histological sections Blumhagen and Coombs (l98l) were able to show that the hypoechoic ring coresponded to the muscular layer in the wall, while the more echogenic central core was formed by the mucosal and submucosal layers, including the muscularis mucosae. On these views the overall diameter of the ring, as well as the thickness of the sonolucent muscular layer, could be measured. The width of the hypoechoic muscular layer in normal infants between the ages of 2 and 10 weeks, was found to be approximately 3.0 mm.

Fig. 10.1. Transverse ultrasonic section of normal pyloric ring showing "doughnut" (arrow). The hypoechoic ring is the muscular, and the inner echogenic core the mucosal/submucosal component of the ring

Strauss et al. (l98l), using a static gray-scale B-scan unit and subsequently a real-time unit with a 5MHz focused transducer, considered the infantile pylorus to be within normal limits if its overall diameter measured 1.5cm or less.

Longitudinal sections of the normal pylorus, on which the canal length can be measured, may also be obtained. In tracing the thin, hypoechoic muscular layer distally to the gastric outlet, Blumhagen and Noble (l983) found the "antral" muscular layer to vary from 1.5 to 3.0 mm in thickness in normal infants. Khamapirad and Athey (l983), using digital gray-scale static equipment and a 5MHz focused transducer, studied transverse and longitudinal sonographic images of the pylorus in 12 normal infants between the ages of one and 6 weeks. The normal pyloric ring was similar in appearance to the mass of IHPS but was less than 1.0cm in diameter.

In a control group of 24 normal infants ranging in age from 2 days to 32 weeks, Graif et al. (l984) found the mean and standard deviation for the transverse diameter to be 7.45 ą 2.2 mm. The mean single wall thickness was 2.3 mm with a standard deviation of ą 0.7 mm, while the mean length of the pylorus was 12.0 mm with a standard deviation of ą 3.7 mm.

Wilson and Vanhoutte (l984) measured what they called the true pyloric muscle length in l7 normal babies, and found the range to vary from 12.0 to 15.0 mm.

Stunden et al. (l986), in 88 normal infants under the age of 5 months, found the mean overall diameter of the pylorus to be 9.1 ą 1.1mm, the mean muscle thickness 1.6 ą 0.4 mm, and the mean canal length 8.3 ą 2.5 mm. These measurements were all obtained with the pylorus in its most contracted state. Additional information could be obtained when the pylorus was viewed in real-time. Normally the pyloric "canal" was seen to relax, allowing fluid to pass from stomach to duodenum. Some variation in overall diameter and muscle thickness did occur, probably representing alterations in muscle tone.

According to Stringer et al. (l986) the thickness of the inner echogenic layer, consisting of the mucosa, muscularis mucosae and submucosa, normally varies between 2.5 and 3.5 mm in infants.

Swischuk (l989) found the hypoechoic outer muscular layer to measure only 1.0 mm in normal infants. At times it was too thin to be measureable.

Discussion

The anatomy of the pyloric ring may be determined accurately by means of non-ionizing, non-invasive sonography in normal human subjects. This has been done in a number of volunteers, in whom it was confirmed that the ring consists mainly of mucosal and submucosal tissues, surrounded by a relatively thin, peripheral muscular ring (Chap. 11). The relative paucity of muscular fibres in the pyloric ring suggests that it is unlikely to be a sphincter in the usually accepted sense of the word.